Chapter 2 / Atomic Structure and Interatomic Bonding

Learning Objectives
After careful study of this chapter you should be able to do the following:
1. Name the two atomic models cited, and note the
differences between them.
2. Describe the important quantum-mechanical
principle that relates to electron energies.
3. (a) Schematically plot attractive, repulsive, and
net energies versus interatomic separation
for two atoms or ions.
2.1 INTRODUCTION
ATOMIC STRUCTURE
2.2 FUNDAMENTAL CONCEPTS
10
(b) Note on this plot the equilibrium separation
and the bonding energy.
4. (a) Briefly describe ionic, covalent, metallic, hydrogen,
and van der Waals bonds.
(b) Note what materials exhibit each of these
bonding types.
Some of the important properties of solid materials depend on geometrical atomic
arrangements, and also the interactions that exist among constituent atoms or
molecules. This chapter, by way of preparation for subsequent discussions, considers
several fundamental and important concepts, namely: atomic structure, electron
configurations in atoms and the periodic table, and the various types of primary
and secondary interatomic bonds that hold together the atoms comprising a solid.
These topics are reviewed briefly, under the assumption that some of the material
is familiar to the reader.
Each atom consists of a very small nucleus composed of protons and neutrons,
which is encircled by moving electrons. Both electrons and protons are electrically
charged, the charge magnitude being 1.60 10 19 C, which is negative in sign for
electrons and positive for protons; neutrons are electrically neutral. Masses for
these subatomic particles are infinitesimally small; protons and neutrons have approximately
the same mass, 1.67 10 27 kg, which is significantly larger than that
of an electron, 9.11 10 31 kg.
Each chemical element is characterized by the number of protons in the nucleus,
or the atomic number (Z). 1 For an electrically neutral or complete atom, the atomic
number also equals the number of electrons. This atomic number ranges in integral
units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring
elements.
The atomic mass (A) of a specific atom may be expressed as the sum of the
masses of protons and neutrons within the nucleus. Although the number of protons
is the same for all atoms of a given element, the number of neutrons (N) may be
variable. Thus atoms of some elements have two or more different atomic masses,
which are called isotopes. The atomic weight of an element corresponds to the
weighted average of the atomic masses of the atom’s naturally occurring isotopes. 2
The atomic mass unit (amu) may be used for computations of atomic weight. A
scale has been established whereby 1 amu is defined as of the atomic mass of
1 Terms appearing in boldface type are defined in the Glossary, which follows Appendix E.
2 The term ‘‘atomic mass’’ is really more accurate than ‘‘atomic weight’’ inasmuch as, in this
context, we are dealing with masses and not weights. However, atomic weight is, by convention,
the preferred terminology, and will be used throughout this book. The reader should
note that it is not necessary to divide molecular weight by the gravitational constant.